The present invention relates to a process for producing molten metal (which term includes metal alloys), in particular although by no means exclusively iron, from metalliferous feed material, such as ores, partly reduced ores and metal-containing waste streams, in a metallurgical vessel containing a molten bath.
The present invention relates particularly to a molten metal bath-based direct smelting process for producing molten metal from a metalliferous feed material.
The most widely used process for producing molten metal is based on the use of a blast furnace. Solid material is charged into the top of the furnace and molten iron is tapped from the hearth. The solid material includes iron ore (in sinter, lump or pellet form), coke, and fluxes and forms a permeable burden that moves downwardly. Preheated air, which may be oxygen enriched, is injected into the bottom of the furnace and moves upwardly through the permeable bed and generates carbon monoxide and heat by combustion of coke. The result of these reactions is to produce molten iron and slag.
A process that produces iron by reduction of iron ore below the melting point of the iron produced is generally classified as a xe2x80x9cdirect reduction processxe2x80x9d and the product is referred to as DRI.
The FIOR (Fluid Iron Ore Reduction) process is an example of direct reduction process. The process reduces iron ore fines as the fines are gravity-fed through each reactor in a series of fluid bed reactors. The fines are reduced by compressed reducing gas that enters the bottom of the lowest reactor in the series and flows counter-current to the downward movement of fines.
Other direct reduction processes include moving shaft furnace-based processes, static shaft furnace-based processes, rotary hearth-based processes, rotary kiln-based processes, and retort-based processes.
The COREX process produces molten iron directly from coal without the blast furnace requirement of coke. The process includes 2-stage operation in which:
(a) DRI is produced in a shaft furnace from a permeable bed of iron ore (in lump or pellet form), coal and fluxes; and
(b) the DRI is then charged without cooling into a connected melter gasifier.
Partial combustion of coal in the fluidised bed of the melter gasifier produces reducing gas for the shaft furnace.
Another known group of processes for producing molten iron is based on cyclone converters in which iron ore is melted by combustion of oxygen and reducing gas in an upper melting cyclone and is smelted in a lower smelter containing a bath of molten iron. The lower smelter generates the reducing gas for the upper melting cyclone.
A process that produces molten metal directly from ores is generally referred to as a xe2x80x9cdirect smelting processxe2x80x9d.
One known group of direct smelting processes is based on the use of electric furnaces as the major source of energy for the smelting reactions.
Another known direct smelting process, which is generally referred to as the Romelt process, is based on the use of a large volume, highly agitated slag bath as the medium for smelting top-charged metal oxides to metal and for post-combusting gaseous reaction products and transferring the heat as required to continue smelting metal oxides. The Romelt process includes injection of oxygen enriched air or oxygen into the slag via a lower row of tuyeres to provide slag agitation and injection of oxygen into the slag via an upper row of tuyeres to promote post-combustion. In the Romelt process the metal layer is not an important reaction medium.
Another known group of direct smelting processes that are slag-based is generally described as xe2x80x9cdeep slagxe2x80x9d processes. These processes, such as DIOS and AISI processes, are based on forming a deep layer of slag with 3 regions, namely: an upper region for post-combusting reaction gases with injected oxygen; a lower region for smelting metal oxides to metal; and an intermediate region which separates the upper and lower regions. As with the Romelt process, the metal layer below the slag layer is not an important reaction medium.
Another known direct smelting process which relies on a molten metal layer as a reaction medium, and is generally referred to as the HIsmelt process, is described in International application PCT/AU96/00197 (WO 96/31627) in the name of the applicant.
The HIsmelt process as described in the International application comprises:
(a) forming a bath of molten iron and slag in a vessel;
(b) injecting into the bath:
(i) metalliferous feed material, typically metal oxides; and
(ii) a solid carbonaceous material, typically coal, which acts as a reductant of the metal oxides and a source of energy; and
(c) smelting the metalliferous feed material to metal in the metal layer.
The HIsmelt process also comprises post-combusting reaction gases , such as CO and H2, released from the bath in the space above the bath with oxygen-containing gas and transferring the heat generated by the post-combustion to the bath to contribute to the thermal energy required to smelt the metalliferous feed materials.
The HIsmelt process also comprises forming a transition zone above the nominal quiescent surface of the bath in which there is a favourable mass of ascending and thereafter descending droplets or splashes or streams of molten metal and/or slag which provide an effective medium to transfer to the bath the thermal energy generated by post-combusting reaction gases above the bath.
The HIsmelt process as described in the International application is characterised by forming the transition zone by injecting a carrier gas and metalliferous feed material and/or solid carbonaceous material and/or other solid material into the bath through a section of the side of the vessel that is in contact with the bath and/or from above the bath so that the carrier gas and the solid material penetrate the bath and cause molten metal and/or slag to be projected into the space above the surface of the bath.
The HIsmelt process as described in the International application is an improvement over earlier forms of the HIsmelt process which form the transition zone by bottom injection of gas and/or carbonaceous material into the bath which causes droplets and splashes and streams of molten metal and slag to be projected from the bath.
This applicant has carried out extensive pilot plant work on the HIsmelt process and has made a series of significant findings in relation to the process.
In general terms, the present invention provides a direct smelting process for producing metals from a metalliferous feed material which includes the steps of:
(a) forming a molten bath having a metal layer and a slag layer on the metal layer in a metallurgical vessel;
(b) injecting metalliferous feed material and solid carbonaceous material into the metal layer via a plurality of lances/tuyeres;
(c) smelting metalliferous material to metal in the metal layer;
(d) causing molten material to be projected as splashes, droplets, and streams into a space above a nominal quiescent surface of the molten bath to form a transition zone; and
(e) injecting an oxygen-containing gas into the vessel via one or more than one lance/tuyere to post-combust reaction gases released from the molten bath, whereby the ascending and thereafter descending splashes, droplets and streams of molten material in the transition zone facilitate heat transfer to the molten bath, and whereby the transition zone minimises heat loss from the vessel via the side walls in contact with the transition zone;
and includes the step of controlling the process by maintaining a high slag inventory.
The invention further provides a direct smelting process for producing metals from a metalliferous feed material which includes the steps of:
(a) forming a molten bath having a metal layer and a slag layer on the metal layer in a metallurgical vessel having side walls, said slag layer providing a high inventory in the vessel;
(b) injecting metalliferous feed material and solid carbonaceous material into the metal layer via a plurality of lances/tuyeres;
(c) smelting metalliferous material to metal in the metal layer;
(d) generating a gas flow from the metal layer at a flow rate of at least 0.04 Nm3/s/m2 of the metal layer area at the interface between the metal layer and the slag layer (under quiescent conditions), which gas flow causes molten material to be projected as splashes, droplets, and streams into a space above a nominal quiescent surface of the molten bath to form a transition zone, the molten material and particularly the slag of the transition zone continuously splashing the side walls and reducing heat loss via the side walls; and
(e) injecting an oxygen-containing gas into the vessel via one or more than one lance/tuyere to post-combust reaction gases released from the molten bath, whereby the ascending and thereafter descending splashes, droplets and streams of molten material in the transition zone facilitate heat transfer to the molten bath, and whereby the transition zone minimises heat loss from the vessel via the side walls in contact with the transition zone.
The above-described gas flow rate from the metal layer of at least 0.04 Nm3/s/m2 area of the metal layer is a substantially higher bath-derived gas flow rate than the rate recommended to successfully operate the Romelt, DIOS, and AISI processes as described above and is a significant difference between the process of the present invention and these known direct smelting processes.
The term xe2x80x9csmeltingxe2x80x9d is understood herein to mean thermal processing wherein chemical reactions that reduce the metalliferous feed material take place to produce liquid metal.
The term xe2x80x9cquiescent surfacexe2x80x9d in the context of the molten bath is understood to mean the surface of the molten bath under process conditions in which there is no gas/solids injection and therefore no bath agitation.
The space above the nominal quiescent surface of the molten bath is hereinafter referred to as the xe2x80x9ctop spacexe2x80x9d.
A significant outcome of the pilot plant work is that it is important to maintain high levels of slag in the vessel (and more particularly in the transition zone) and to generate sufficient gas flow from the metal layer such that the side walls are continuously splashed by molten material, particularly slag, in the transition zone in order to control heat losses from the vessel. The importance of slag to the HIsmelt process is a significant departure from previous work on the HIsmelt process. In the previous work the amount of slag was not considered to be as important to the process.
Preferably the gas flow rate is at least 0.2 Nm3/s/m2 of the metal layer area.
Preferably the gas flow rate is less than 2 Nm3/s/m2.
The gas flow from the metal layer may be caused by any one or more of a number of factors. For example, gas flow may be generated at least in part as a result of injection of metalliferous feed material and solid carbonaceous material into the metal layer. By way of further example, the gas flow may be generated at least in part as a result of injection of a carrier gas into the metal layer with injected metalliferous feed material and/or solid carbonaceous material. By way of further example, gas flow may be generated at least in part as a result of bottom and/or side wall injection of a gas into the metal layer.
Preferably the gas flow rate of at least 0.04 Nm3/s/m2 of the metal layer area forms a metal-rich transition zone above the metal layer and a slag-rich transition zone above the metal-rich transition zone.
The concept of a xe2x80x9chigh slag inventoryxe2x80x9d may be understood in the context of the depth of the slag layer in the vessel.
Preferably the process includes maintaining the high slag inventory by controlling the slag layer to be 0.5 to 4 meters deep under stable operating conditions.
More preferably the process includes maintaining the high slag inventory by controlling the slag layer to be 1.5 to 2.5 meters deep under stable operating conditions.
It is preferred particularly that the process includes maintaining the high slag inventory by controlling the slag layer to be at least 1.5 meters deep under stable operating conditions.
The concept of a xe2x80x9chigh slag inventoryxe2x80x9d may also be understood in the context of the amount of slag compared to the amount of metal in the vessel.
Preferably, when the process is operating under stable conditions, the process includes maintaining the high slag inventory by controlling the weight ratio of metal:slag to be between 4:1 and 1:2.
More preferably the process includes maintaining the high slag inventory by controlling the weight ratio of metal:slag to be between 3:1 and 1:1.
It is preferred particularly that the process includes maintaining the high slag inventory by controlling the metal:slag weight ratio to be between 3:1 and 2:1.
The amount of slag in the vessel, ie the slag inventory, has a direct impact on the amount of slag that is in the transition zone.
The relatively low heat transfer characteristics of slag compared to metal is important in the context of minimising heat loss from the transition zone to the side walls and from the vessel via the side walls of the vessel.
By appropriate process control, slag in the transition zone can form a layer or layers on the side walls that adds resistance to heat loss from the side walls.
Therefore, by changing the slag inventory it is possible to increase or decrease the amount of slag in the transition zone and on the side walls and therefore control the heat loss via the side walls of the vessel.
The slag may form a xe2x80x9cwetxe2x80x9d layer or a xe2x80x9cdryxe2x80x9d layer on the side walls. A xe2x80x9cwetxe2x80x9d layer comprises a frozen layer that adheres to the side walls, a semi-solid (mush) layer, and an outer liquid film. A xe2x80x9cdryxe2x80x9d layer is one in which substantially all of the slag is frozen.
The amount of slag in the vessel also provides a measure of control over the extent of post combustion.
Specifically, if the slag inventory is too low there will be increased exposure of metal in the transition zone and therefore increased oxidation of metal and dissolved carbon in metal and the potential for reduced post-combustion and consequential decreased post combustion, notwithstanding the positive effect that metal in the transition zone has on heat transfer to the metal layer.
In addition, if the slag inventory is too high the one or more than one oxygen-containing gas injection lance/tuyere will be buried in the transition zone and this minimises movement of top space reaction gases to the end of the or each lance/tuyere and, as a consequence, reduces potential for post-combustion.
The amount of slag in the vessel, ie the slag inventory, measured in terms of the depth of the slag layer or the weight ratio of metal:slag, may be controlled by the tapping rates of metal and slag.
The production of slag in the vessel may be controlled by varying the feed rates of metalliferous feed material, carbonaceous material, and fluxes to the vessel and operating parameters such as oxygen-containing gas injection rates.
The process of the present invention is characterised by controlling heat transfer via the transition zone to the metal layer and controlling heat loss from the vessel via the transition zone.
As noted above, in particular the present invention is preferably characterised by controlling the process by maintaining a high slag inventory.
In addition, the present invention is more preferably characterised by controlling the process by means of the following process features, separately or in combination;
(a) locating the one or more than one oxygen-containing gas injection lance/tuyere and injecting the oxygen-containing gas at a flow rate so that:
(i) the oxygen-containing gas is injected towards the slag layer and penetrates the transition zone; and
(ii) the stream of oxygen-containing gas deflects the splashes, droplets and streams of molten material around a lower section of the or each lance/tuyere and a gas continuous space described as a xe2x80x9cfree spacexe2x80x9d forms around the end of the or each lance/tuyere;
(b) controlling heat loss from the vessel by splashing predominantly slag onto the side walls of the vessel in contact with the transition zone by adjusting one or more of:
(i) the amount of slag in the molten bath;
(ii) the injection flow rate of the oxygen-containing gas through the one or more than one oxygen-containing gas injection lance/tuyere; and
(iii) the flow rate of metalliferous feed material and carbonaceous material through the lances/tuyeres.
In situations where the metalliferous feed material is an iron-containing material, the present invention is also preferably characterised by controlling the process by controlling the level of dissolved carbon in molten iron to be at least 3 wt % and maintaining the slag in a strongly reducing condition leading to FeO levels of less than 6 wt %, more preferably less than 5 wt %, in the slag layer and in the transition zone.
Preferably, the metallurgical vessel includes:
(a) the above-described one or more than one lance/tuyere for injecting oxygen-containing gas and the lances/tuyeres for injecting solid materials, such as metalliferous material, carbonaceous material (typically coal) and fluxes, into the vessel;
(b) outlets for discharging molten metal and slag from the vessel; and
(c) one or more off-gas outlet.
In order to operate the process it is essential that the vessel contains a molten bath having a metal layer and a slag layer on the metal layer.
The term xe2x80x9cmetal layerxe2x80x9d is understood herein to mean that region of the bath that is predominantly metal.
The term xe2x80x9cslag layerxe2x80x9d is understood herein to mean that region of the bath that is predominantly slag.
An important feature of the process of the present invention is that metalliferous material is smelted to metal at least predominantly in the metal layer of the molten bath.
In practice, there will be a proportion of the metalliferous material that is smelted to metal in other regions of the vessel, such as the slag layer. However, the objective of the process of the present invention, and an important difference between the process and prior art processes, is to maximise smelting of metalliferous material in the metal layer.
As a consequence of the above, the process includes injecting metalliferous material and carbonaceous material, which acts as a source of reductant and as a source of energy, into the metal layer.
One option is to inject metalliferous material and carbonaceous material via lances/tuyeres positioned above and extending downwardly towards the metal layer. Typically, the lances/tuyeres extend through side walls of the vessel and are angled inwardly and downwardly towards the surface of the metal layer.
Another option, although by no means not the only other option, is to inject metalliferous material and carbonaceous material via tuyeres in the bottom of the vessel or in side walls of the vessel that contact the metal layer.
The injection of metalliferous material and carbonaceous material may be through the same or separate lances/tuyeres.
Another important feature of the process of the present invention is that it causes molten material, typically in the form of splashes, droplets, and streams, to be projected upwardly from the molten bath into at least part of the top space above the quiescent surface of the bath to form the transition zone.
The transition zone is quite different to the slag layer. By way of explanation, under stable operating conditions of the process the slag layer comprises gas bubbles in a liquid continuous volume whereas the transition zone comprises splashes, droplets, and streams of molten material in a gas continuous volume.
Preferably the process includes causing molten material to be projected as splashes, droplets and streams into the top space above the transition zone.
Another important feature of the present invention is that it post-combusts reaction gases, such as carbon monoxide and hydrogen, generated in the molten bath, in the top space (including the transition zone) above the nominal quiescent surface of the bath and transfers the heat generated by the post-combustion to the metal layer to maintain the temperature of the molten bathxe2x80x94as is essential in view of endothermic reactions in that layer.
Preferably the oxygen-containing gas is air.
More preferably the air is pre-heated.
Typically, the air is preheated to 1200xc2x0 C.
The air may be oxygen enriched.
Preferably the level of post-combustion is at least 40%, where post-combustion is defined as:             [              CO        2            ]        +          [                        H          2                ⁢        O            ]                  [              CO        2            ]        +          [                        H          2                ⁢        O            ]        +          [      CO      ]        +          [              H        2            ]      
where:
[CO2]=volume % of CO2 in off-gas
[H2O]=volume % of H2O in off-gas
[CO]=volume % of CO in off-gas
[H2]=volume % of H2 in off-gas
The transition zone is important for 2 reasons.
Firstly, the ascending and thereafter descending splashes, droplets and streams of molten material are an effective means of transferring to the molten bath the heat generated by post-combustion of reaction gases in the top space above the quiescent surface of the bath.
Secondly, the molten material, and particularly the slag, in the transition zone is an effective means of minimising heat loss via the side walls of the vessel.
A fundamental difference between the process of the present invention and prior art processes is that in the process of the present invention the main smelting region is the metal layer and the main oxidation (ie heat generation) region is above and in the transition zone and these regions are spatially well separated and heat transfer is via physical movement of molten metal and slag between the two regions.
Preferably the transition zone is generated by injecting metalliferous material and carbonaceous material in a carrier gas through lances/tuyeres that extend downwardly towards the metal layer.
More preferably, as noted above, lances/tuyeres extend through the side walls of the vessel and are angled inwardly and downwardly towards the metal layer.
This injection of the solid material towards and thereafter into the metal layer has the following consequences:
(a) the momentum of the solid material/carrier gas causes the solid material and gas to penetrate the metal layer;
(b) the carbonaceous material, typically coal, is devolatilised and thereby produces gas in the metal layer;
(c) carbon predominantly dissolves into the metal and partially remains as solid;
(d) the metalliferous material is smelted to metal by carbon derived from injected carbon as described above in item (c) and the smelting reaction generates carbon monoxide gas; and
(e) the gases transported into the metal layer and generated via devolatilisation and smelting produce significant buoyancy uplift of molten metal, solid carbon and slag (which is drawn into the metal layer as a consequence of solid/gas injection) from the metal layer which results in upward movement of splashes, droplets and streams of molten metal and slag, and these splashes, droplets, and streams entrain further slag as they move through the slag layer.
Another important feature of the present invention is that the location and operating parameters of the one or more than one lance/tuyere that injects the oxygen-containing gas and the operating parameters that control the transition zone are selected so that:
(a) the oxygen-containing gas is injected towards the slag layer and penetrates the transition zone;
(b) the stream of oxygen-containing gas deflects the splashes, droplets and streams of molten material so that, in effect:
(i) the transition zone extends upwardly around the lower section of the one or more than one lance/tuyere; and
(ii) a gas continuous space described as a xe2x80x9cfree spacexe2x80x9d forms around the end of the one or more than one lance/tuyere.
The formation of the free space is an important feature because it makes it possible for reaction gases in the top space of the vessel to be drawn into the region of the end of the one or more than one oxygen-containing gas injection lance/tuyere and to be post-combusted in the region. In this context, the term xe2x80x9cfree spacexe2x80x9d is understood to mean a space which contains practically no metal and slag.
In addition, the above-described deflection of molten material shields to some degree the side walls of the vessel from the combustion zone generated at the end of the or each lance/tuyere. Also it provides a means for returning more energy back to the bath from gases post combusted in the top space.
Preferably the process includes injecting the oxygen-containing gas into the vessel in a swirling motion.